1. Field of the Invention
The present invention generally relates to a two-stroke internal combustion engine and, more particularly, to an engine of a reverse scavenging type.
2. Description of the Related Art
Since the two-stroke internal combustion engine includes a small number of components and is small in size and weight, the two-stroke internal combustion engine is applied to portable or handheld working machines such as a brush cutter, a blower, and a chain saw. The two-stroke internal combustion engine of this type includes a scavenging passage connected to a cylinder chamber and a crank chamber. The scavenging passage is open to the cylinder chamber through a scavenging port. The scavenging port is opened and closed by a piston. As it is well known, the cylinder chamber is defined by a piston. A crankshaft is housed in the crank chamber, and a reciprocating motion of the piston is converted into a rotary motion by the crankshaft.
WO98/57053 (Patent Document No. 1) discloses a two-stroke internal combustion engine that uses fresh air for scavenging. The two-stroke internal combustion engine of this type is called “stratified scavenging type engine”. Patent Document No. 1 discloses various stratified scavenging type engines. Specifically, the stratified scavenging type engine includes a gas mixture passage and a fresh air passage as an intake system of the engine. A gas mixture passing through the gas mixture passage is introduced into the crank chamber through a gas mixture port opened and closed by a piston.
The stratified scavenging type engine is classified into two types according to methods of introducing the fresh air into the scavenging passage. A first engine is an engine of a reed valve type. A second engine is an engine of a piston groove type. Patent Document No. 1 discloses the piston groove type engine.
The reed valve type engine includes a reed valve that controls the fresh air charged in the scavenging passage. A position of the reed valve in the engine will be explained. The fresh air passage merges with the scavenging passage in an upper part of the scavenging passage, that is, in a position near the scavenging port. The reed valve is arranged in this merging portion. In the reed valve type engine, when the piston ascends and the pressure in the crank chamber falls, the gas mixture flows into the crank chamber through the gas mixture passage and the reed valve opens. When the reed valve opens, the fresh air is fed from the fresh air passage to the scavenging passage.
When the gas mixture burns in the cylinder chamber, the piston descends with the combustion pressure in the cylinder chamber and the pressure in the crank chamber ascends. Halfway in the descent of the piston, that is, before the piston reaches the bottom dead center (BDC), an exhaust port opens according to the descent of the piston and, subsequently, the scavenging port opens. When the exhaust port opens, a combustion gas is discharged through the exhaust port. In a scavenging stroke in which the scavenging port opens, the fresh air accumulated in the upper part of the scavenging passage spouts to the cylinder chamber. The combustion gas remaining in the cylinder chamber is forced out to the outside through the exhaust port by the fresh air. In other words, the scavenging process is performed by the fresh air flowing into the cylinder chamber through the scavenging port.
The piston groove type engine includes a piston groove, through which the fresh air passes, on the outer circumferential surface of the piston. When the piston is located in a predetermined height position by moving up and down, the piston groove communicates with the fresh air passage and the scavenging port. The fresh air is fed from the fresh air passage to the scavenging passage via the piston groove by the communication. When the piston ascends and the pressure in the crank chamber falls, the gas mixture flows into the crank chamber through the gas mixture passage. The fresh air passage and the scavenging passage also communicate with each other through the piston grove, so that the fresh air flows into the scavenging passage.
Other precedents concerning the piston groove type engine are cited. U.S. Pat. No. 7,082,910 B2 (Patent Document No. 2), U.S. Pat. No. 7,565,886 B2 (Patent Document No. 3), and Japanese Patent Laid-Open No. 2001-173447 (Patent Document No. 4) disclose techniques for causing the fresh air passage and the scavenging passage to communicate with each other through the piston groove.
As a scavenging method for the two-stroke internal combustion engine, a “reverse scavenging” method is well known. Japanese Patent Laid-Open No. 60-222522 (Patent Document No. 5) discloses an engine of a Schnurle type, which is a typical example of the “reverse scavenging” method. Specifically, the Schnurle type engine disclosed in Patent Document No. 5 includes a pair of scavenging passages on the left and right when a cylinder bore is viewed in plan view. Scavenging ports of the respective scavenging passages are directed to the opposite side of exhaust ports.
In a scavenging stroke, gas spouted from the scavenging port into the cylinder chamber is directed in a direction away from the exhaust port. Then, the gas collides against a wall surface of the cylinder bore located on the opposite side of the exhaust port to be reversed and directed to the exhaust port. The “reversed scavenging method has an effect of, for example, suppressing so-called “blow-by” in which the gas mixture passes through the cylinder chamber without staying therein and is emitted to the outside from the exhaust port in a scavenging stroke.
Representative effects of the reverse scavenging type engine are illustratively listed below.    (1) “Blow-by of the gas mixture” is small in which a new gas mixture introduced into the cylinder chamber passes through the cylinder chamber without staying in the cylinder chamber. Therefore, scavenging efficiency is high and it is possible to improve a fuel consumption ratio.    (2) It is possible to reduce an amount of HC in an exhaust gas (improvement of emission).    (3) Since a plurality of pairs of scavenging ports can be provided, it is possible to expand a total capacity of the scavenging passages. Incidentally, Patent Document No. 5 discloses an engine including three pairs of scavenging ports.
Patent Document No. 5 discloses an invention having an object of reducing a blow-by loss of a gas mixture. The invention relates to a scavenging passage and proposes shaping of a wall surface of the scavenging passage into a specific shape. Specifically, the invention proposes a structure concerning a wall surface of a scavenging passage portion adjacent to a scavenging port. More specifically, when a cylinder chamber is viewed in plan view, according to the invention of Patent Document No. 5, it is proposed that a wall surface on a side close to an exhaust port in the wall surface of the scavenging passage portion adjacent to the scavenging port is composed of an inclined surface. According to the invention, a flowing direction of a scavenging gas passing through the scavenging passage is directed to the opposite side of the exhaust port near the scavenging port by the inclined surface. Consequently, a flow of the scavenging gas spouting from the scavenging port to the cylinder chamber is directed to a direction away from the exhaust port.
The blow-by loss of the gas mixture is an important technical problem considered to be a fate of the two-stroke internal combustion engine. Improvement of the blow-by loss of the gas mixture is directly linked to improvement of a fuel consumption ratio and improvement of emission. In particular, the recent environmental problem requests further improvement of the blow-by loss.
Therefore, it is an object of the present invention to provide a two-stroke internal combustion engine that can reduce the blow-by loss of the gas mixture.